The aim of the research is to investigate the influence of focal visual attention upon sensory processes at several stages of the cortical visual system in primates. The research plan is to compare the neural activity evoked by physically identical test stimuli presented under different attentional conditions in combined neurophysiological and psychophysical investigations. The major objective is to determine whether attentional mechanisms exert dynamic control over the neural representation of visual events in the initial visual cortical areas. Combined electrophysiological and behavioral investigations will be made of the influence of focal attention of the processing of visual information about oriented line segments in three early stages of cortical visual procesisng (V1, V2, and V3/V3A). Two approaches will be used. The first is to compare the response properties of neurons in the respective visual cortical areas under conditions in which the locus of focal attention is controlled and is directed either at the target stimulus or away from the target stimulus. The second approach is to compare the response properties and profiles under conditions in which focal attention is required to identify the target stimulus and, by contrast, under those conditions in which the physically indentical target stimulus can be identified without focal attention. The objectives are to determine the influence of attention upon the early cortical processing of visual afferent input, to determine the nature of the changes, to determine where in the chain of cortical processing the changes occur, and to determine when they occur in time relative to the stimulus event.